[PATCH] drm/i915: Avoid flicker with horizontal panning on 830GM

Thomas Richter
2013-09-01T17:02:11Z

Dear intel-gfx developers,

When panning is enabled on the 830GM, horizontal panning creates a lot
of flickering on specific pixel positions.
After testing, I found that the reason for this is that panning works by
altering the frame origin pointer, which,
however, has certain alignment restrictions. If the pointer is not
aligned correctly, the screen starts to flicker
as, probably, DMA fails.

The following patch against drm/i915/intel_display.c fixes the issue by
ensuring correct alignment. As result,
horizontal panning works correctly, but is a bit "jumpy". Unclear
whether the problem affects any other
chipset revisions, thus the patch is currently only enabled for rev.2.

Greetings,
Thomas

diff --git a/drivers/gpu/drm/i915/intel_display.c
b/drivers/gpu/drm/i915/intel_display.c
index bcb62fe..8304e30 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -1987,8 +1987,13 @@ static int i9xx_update_plane(struct drm_crtc
*crtc, struct drm_framebuffer *fb,

I915_WRITE(reg, dspcntr);

- linear_offset = y * fb->pitches[0] + x * (fb->bits_per_pixel / 8);
-
+ if (INTEL_INFO(dev)->gen > 2) {
+ linear_offset = y * fb->pitches[0] + x * (fb->bits_per_pixel / 8);
+ } else {
+ /* align the linear offset to 64 pixel boundaries */
+ linear_offset = y * fb->pitches[0] + (x & -32) *
(fb->bits_per_pixel / 8);
+ }
+
if (INTEL_INFO(dev)->gen >= 4) {
intel_crtc->dspaddr_offset =
intel_gen4_compute_page_offset(&x, &y,
obj->tiling_mode,